The present invention generally relates to the field of antennas, and more particularly to a retractable helical antenna designed for use with miniature portable radio transceivers.
Until recently, two-way portable radios have primarily utilized monopole antennas, which are usually deployed from a retracted storage position to an extended operating position. For frequencies in the VHF range (30 MHz. to 300 MHz.), a monopole antenna must be extended on the order of two feet to efficiently transmit. Not only is this antenna-deploying procedure inconvenient to the user, but also possibly dangerous under some circumstances. Moreover, with the continuing trend to make portable radio equipment smaller, there has been a corresponding interest in size-reduction for portable antennas. For example, in a portable "shirt-pocket" radio, where the entire radio case measures only five inches in height, a two-foot antenna is considered highly impracticable.
These reasons illustrate why helical antennas have now become very popular antenna configurations for portable radios. The helical shape of the antenna is attractive for mechanical reasons, since it generally requires only 1/10th of the height of a monopole at the same frequency. Additionally, the helical antenna provides excellent electrical characteristics, such as efficiencies on the order of 60%. Furthermore, some helical antennas are easily compressed into even a smaller size for storage. A collapsible configuration is described in U.S. Pat. No. 3,836,979 for an axial mode helical antenna.
Helical antennas are operated in different modes for different applications. To obtain the most compact antenna, the helix is operated in the normal mode. In the normal radiation mode, the diameter of the helix is a small fraction of the wavelength and the electrical length is less than one wavelength. Typically, portable radio helical antennas have an electrical length of less than one-fourth wavelength. However, in the normal mode, the frequency bandwidth of the helical antenna is quite narrow. Hence, the potential uses for helical antennas have previously been limited to applications where a narrow bandwidth is acceptable, such as simplex (single-frequency) radio systems.
This frequency bandwidth limitation of helical antennas have had a significant impact on the size-vs.-performance tradeoff of portable radio design. Portables often operate through repeaters for a wide-area coverage. In such repeater applications, these portable radios transmit on one frequency and receive on another, usually widely-spaced from the first. The wide Tx/Rx frequency spacing necessitates that a performance compromise be made for helical antennas--between optimal antenna efficiency at the transmit or receive frequency. In the alternative, a dual antenna configuration, such as the monopole/helix arrangement described in U.S. Pat. No. 4,121,218, may be provided. However, this approach contradicts the size-minimization and cost-reduction goals of most portable products.
Another approach to the size/performance problem of helical antennas is to tune the antenna over the desired frequency range by changing the fraction of the total helix used as the antenna portion. This can be accomplished by either shorting-out the unused portion of the helix via sliding contacts as shown in U.S. Pat. No. 4,087,820, or by varying the number of turns in the expanded section of the helix, as described in U.S. Pat. No. 3,858,220. Both of these prior art antennas have mechanical limitations which make it very difficult to implement and highly unattractive for use with miniature portable transceivers at VHF frequencies. Moreover, these prior methods of tuning helical antennas would prove to be too awkward and intricate for portable radio applications requiring repeated tuning to widely-spaced transmit and receive frequencies.
Therefore, a need exists for an antenna which can be easily tuned to frequency and readily adapted to portable radio transceiver applications.